How can I detect a held down mouse button over a PictureBox?

Question

I need to fire an event when the mouse is above a PictureBox with the mouse button already clicked and held down.

Problems:

The MouseDown and MouseEnter event handlers do not work together very well.

For instance once a mouse button is clicked and held down, C# will fire the MouseDown event handler, but when the cursor moves over the PictureBox the MouseEnter event does not fire, until the mouse button is realeased.

Answer 1

set a flag or a state on mouse down. release it on mouse up. When on mouse over fires for the picture box check your state. Now you can detect when a person is dragging something.

Answer 2

Mouse events

Use the MouseDown event to just detect a down press of a mouse button and set this.Capture to true so that you then get other mouse events, even when the mouse leaves the control (i.e. you won't get a MouseLeave event because you captured the mouse). Release capture by setting this.Capture to false when MouseUp occurs.

Just checking the state of the mouse

This may not be relevant, but you can check System.Windows.Control.MousePosition and see if it is in the PictureBox.ClientRectangle, then check the Control.MouseButtons static property for which buttons might be down at any time.

As in:

if  (pictureBox.ClientRectangle.Contains(pictureBox.PointToClient(Control.MousePosition)))
{
   if (Control.MouseButtons & MouseButtons.Left) != 0)
   {
     // Left button is down.
   }
}

Answer 3

When the mouse is pressed down most controls will then Control.Capture the mouse input. This means that all MouseMove events are sent to the original control that captured rather than the control the mouse happens to be over. This continues until the mouse loses capture which typically happens on the mouse up.

If you really need to know when the mouse is over your control even when another control has captured mouse input then you only really have one way. You need to snoop the windows messages destined for other controls inside your application. To do that you need add a message filter ...

Application.AddMessageFilter(myFilterClassInstance);

Then you need to implement the IMessageFilter on a suitable class...

public class MyFilterClass : IMessageFilter
{
    public bool PreFilterMessage(ref Message m)
    {
        if (m.Msg == WM_MOUSEMOVE)
            // Check if mouse is over my picture box!

        return false;
    }
}

Then you watch for mouse move events and check if they are over your picture box and do whatever it is you want to do.

Answer 4

If you're trying to implement a drag-and-drop operation of some sort, the Drag... events (DragEnter, DragDrop etc.) on the receiving picture box are what you want to use. Basically, you start the drag operation using the DoDragDrop method of the source control, and then any control that you drag over will have its Drag... events raised.

Search "DoDragDrop" on MSDN to see how to implement this.